1. Field of the Invention
The present invention relates to a so-called MIMO-OFDM wireless communication apparatus particularly useful in a high-speed wireless LAN which performs communication by using a plurality of antennas and a plurality of subcarriers.
2. Description of the Related Art
In the conventional wireless LAN standard, e.g., the IEEE 802.11a, synchronous processing and channel estimation are performed by transmitting known symbols (a short preamble and long preamble) before a data signal. By using these preambles, the subsequent signal portion and data portion can be demodulated.
Recently, a high-speed wireless LAN standard called IEEE 802.11n is being established. To achieve a transmission rate of 100 Mbps in a MAC layer, the IEEE 802.11n is based on multi-input multi-output (MIMO) using a plurality of antennas. In this MIMO technique, known symbols must be transmitted from a plurality of transmitting antennas by using preambles, in order to estimate the channel responses of propagation paths from these transmitting antennas to each receiving antenna.
In a preamble signal plan proposed by Jan Boer et al. in “Backwards Compatibility”, IEEE 802.11-03/714r0, a short preamble sequence used for time synchronization, frequency synchronization, and AGC, a long preamble containing a symbol for channel estimation, and a signal field are first transmitted from one transmitting antenna, and then long preambles for channel estimation are transmitted in order from other transmitting antennas. After the transmission of the preamble signals is thus complete, data is simultaneously transmitted from a plurality of transmitting antennas. That is, long preambles for channel estimation are transmitted from a plurality of transmitting antennas by time-division multiplexing. In this preamble signal plan by Jan Boer et al., each long preamble is transmitted using a single antenna because long preambles containing known symbols for channel estimation are transmitted from a plurality of transmitting antennas by time-division multiplexing. On the other hand, data symbols following these preambles are simultaneously transmitted by using a plurality of antennas.
In a wireless communication apparatus, the transmitting power of a transmitter is desirably constant with time. However, when the numbers of antennas used in known symbol transmission and data symbol transmission are different as described above, the gain of a power amplifier during known symbol transmission must be set higher than that during data symbol transmission in order to hold the transmitting power constant. That is, the gains of the power amplifier must be frequently switched during transmission. This gain switching applies a load on the power amplifier and its control system, and lowers the throughput.
On the other hand, if the power of received signals largely fluctuates in a receiver because the number of antennas used in transmission changes, it becomes difficult to perform automatic gain control (AGC) by which the input signal level of an analog-to-digital converter (ADC) for converting a received signal into a digital signal is controlled within the dynamic range of this ADC.